1. Field of the Invention
The present invention relates to a control device and a control method for an internal combustion engine, and more particularly, to a control device and a control method for an internal combustion engine which is, for example, to be mounted in a supercharged engine vehicle.
2. Description of the Related Art
A turbocharger is conventionally used to improve an output of an internal combustion engine (hereinafter referred to simply as “engine”). The turbocharger is a device including a supercharger provided in an intake path of the engine, which rotates a turbine of the supercharger by using an exhaust gas to forcibly feed air to the engine. In the turbocharger, a supercharging pressure is undesirably increased than needed when the engine rotates at a high speed under a high load, which brings about a risk of breaking the engine. Therefore, in general, an exhaust bypass passage is provided upstream of the turbine. In the exhaust bypass passage, a wastegate valve (hereinafter referred to as “WGV”) is provided. The WGV causes a part of the exhaust gas flowing through an exhaust path to flow into the exhaust bypass passage to adjust an inflow amount of the exhaust gas to the turbine. In this manner, a pressure (supercharging pressure) in an intake path of the engine is controlled at a proper level.
In general, the WGV is driven by using a positive-pressure actuator. The intake path of the engine (in particular, a portion located upstream of a throttle valve, in which a pressure is increased) and a wastegate actuator (hereinafter referred to as “WGA”) are connected to each other. Therefore, when the pressure in the intake path of the engine during a supercharging operation becomes larger than an atmospheric pressure, an operation of the WGA is enabled. By adjusting a relief amount of a relief valve connected to the WGA, a pressure to be supplied to the WGA is adjusted. At the same time, an opening degree of the WGV which cooperates with the WGA is also adjusted. In general, a detector for detecting the amount of actuation is not mounted to the WGA and the WGV. Thus, by using a detected pressure value on a downstream side of a compressor, such as an intake-manifold pressure (hereinafter referred to also as “Pb”), the relief amount of the relief valve is adjusted. In a pressure state before the operation of the WGA is enabled, the WGV which is a bypass valve is retained in a fully-closed state by a mechanical element such as a spring included in the WGA.
The output of the engine is adjusted generally by operating a throttle valve (hereinafter referred to as “THV”) provided in the intake path to adjust an opening area of the intake path. By using the opening area of the THV, an upstream pressure and a downstream pressure of the THV with respect to an air flow, and detection values of an upstream temperature and a downstream temperature of the THV, a flow rate of air passing through the THV can be controlled based on a physical calculation formula. The above-mentioned technology using the physical calculation formula is generally employed (for example, see Japanese Patent No. 4237214). However, the technology described above requires a sensor for measuring the upstream pressure of the THV and therefore, has a disadvantage in that the number of components undesirably increases to increase costs.
In view of the above-mentioned problem, a technology for reducing costs is described in Japanese Patent No. 4583038, for example. In the technology described in Japanese Patent No. 4583038, a P2 sensor for detecting a throttle-valve upstream pressure (hereinafter referred to also as “P2”) is not used. In Japanese Patent No. 4583038, a gas amount passing through the WGV is calculated based on a detection value of an intake-air flow rate and the WGV opening degree. In this manner, a gas amount supplied to the turbine is calculated based on the gas amount passing through the WGV. Then, the P2 is estimated from the gas amount supplied to the turbine.
As described above, the technology described in Japanese Patent No. 4237214 has a problem in that the costs increase because of the use of the P2 sensor.
With the technology described in Japanese Patent No. 4583038, the opening degree of the WGV is required to be estimated in a system without a detector for detecting the WGV opening degree. Therefore, the technology described in Japanese Patent No. 4583038 has a problem in that the estimate value of the P2 is affected due to estimation accuracy for the opening degree of the WGV. On the other hand, when the positive-pressure WGA is used, a driving force of the WGA fluctuates depending on a state of the P2. Therefore, a complex calculation formula is required to be used to estimate the WGV opening degree by using a control value for the WGA and various types of operation information of the engine.